In 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE (hereinafter referred to as LTE)), Orthogonal Frequency Division Multiple Access (OFDMA) is adopted as a downlink communication scheme, and Single Carrier Frequency Division Multiple Access (SC-FDMA) is adopted as an uplink communication scheme (e.g., see NPL-1, NPL-2, and NPL-3).
In LTE, a base station apparatus for radio communications (hereinafter abbreviated as “base station”) performs communications by allocating a resource block (RB) in a system band to a terminal apparatus for radio communications (hereinafter abbreviated as “terminal”) for every time unit called “subframe.” The base station also transmits allocation control information (i.e., L1/L2 control information) for the notification of the result of resource allocation of downlink data and uplink data to the terminal. The allocation control information is transmitted to the terminal through a downlink control channel such as a Physical Downlink Control Channel (PDCCH). A resource region to which a PDCCH is to be mapped is specified. As shown in FIG. 1, a PDCCH covers the entire system bandwidth in the frequency-domain and the region occupied by the PDCCH in the time-domain varies between a leading first OFDM symbol and a third OFDM symbol in a single subframe. A signal indicating a range of OFDM symbols occupied by a PDCCH in the time-domain direction is transmitted through a Physical Control Format Indicator Channel (PCFICH).
Each PDCCH also occupies a resource composed of one or more consecutive control channel elements (CCEs). In a PDCCH, one CCE consists of 36 resource elements (RE). In LTE, the number of CCEs occupied by a PDCCH (CCE aggregation level, or simply aggregation level) is selected from 1, 2, 4, and 8 depending on the number of bits of allocation control information or the condition of a propagation path of a terminal. In LTE a frequency band having a system bandwidth of up to 20 MHz is supported.
Allocation control information transmitted from a base station is referred to as downlink control information (DCI). If a base station allocates a plurality of terminals to one subframe, the base station transmits a plurality of items of DCI simultaneously. In this case, in order to identify a terminal to which each item of DCI is transmitted, the base station transmits the DCI with CRC bits included therein, the bits being masked (or scrambled) with a terminal ID of the transmission destination terminal. Then, the terminal performs demasking (or descrambling) on the CRC bits of a plurality of items of possible DCI directed to the terminal with its own ID, thereby blind-decoding a PDCCH to detect the DCI directed to the terminal.
DCI also includes information (resource allocation information (for example, the number of allocated resource blocks)) regarding a Physical Downlink Shared Channel (PDSCH) resource and a Physical Uplink Shared Channel (PDSCH) resource allocated to a terminal by a base station. In addition, DCI includes a Modulation and channel Coding Scheme (MCS) (for example, information showing an M-ary modulation number and/or a transport block size with respect to the number of allocated resource blocks) allocated to a terminal by a base station and the like. Furthermore, DCI has a plurality of formats for uplink, downlink Multiple Input Multiple Output (MIMO) transmission, and downlink non-consecutive band allocation. A terminal needs to receive both downlink allocation control information (i.e., allocation control information about a downlink) and uplink allocation control information (i.e., allocation control information about an uplink) which have a plurality of formats.
For example, for the downlink allocation control information, formats of a plurality of sizes are defined depending on a method for controlling a transmission antenna of a base station and a method for allocating a resource. Among the formats, a downlink allocation control information format for consecutive band allocation (hereinafter simply referred to as “downlink allocation control information”) and an uplink allocation control information format for consecutive band allocation (hereinafter simply referred to as “uplink allocation control information”) have the same size. These formats (i.e., DCI formats) include type information (for example, a one-bit flag) indicating the type of allocation control information (downlink allocation control information or uplink allocation control information). Thus, even if DCI indicating downlink allocation control information and DCI indicating uplink allocation control information have the same size, a terminal can determine whether specific DCI indicates downlink allocation control information or uplink allocation control information by checking type information included in allocation control information.
The DCI format in which uplink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 0” (hereinafter referred to as “DCI 0”), and the DCI format in which downlink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 1A” (hereinafter referred to as “DCI 1A”). Since DCI 0 and DCI 1A are of the same size and distinguishable from each other by referring to type information as described above, hereinafter, DCI 0 and DCI 1A will be collectively referred to as DCI 0/1A.
In addition to these DCI formats, there are other formats for downlink, such as DCI format 1 used for non-consecutive band allocation (hereinafter referred to as DCI 1) and DCI formats 2 and 2A used for allocating spatial multiplexing MIMO transmission (hereinafter referred to as DCI 2 and 2A). DCI 1, DCI 2, and DCI 2A are formats that are dependent on a downlink transmission mode of a terminal (non-consecutive band allocation or spatial multiplexing MIMO transmission) and configured for each terminal. In contrast, DCI 0/1A is a format that is independent of the transmission mode and can be used for a terminal having any transmission mode, i.e., a format commonly used for every terminal. If DCI 0/1A is used, single-antenna transmission or a transmit diversity scheme is used as a default transmission mode.
Also, the standardization of 3GPP LTE-Advanced (hereinafter referred to as LTE-A), which provides a data transfer rate higher than that of LTE, has been started while having backward compatibility with LTE. In LTE-A, in order to achieve a downlink transfer rate of up to 1 Gbps and an uplink transfer rate of up to 500 Mbps, base stations and terminals (hereinafter referred to as LTE-A terminals) capable of communicating at a wideband frequency of 40 MHz or higher will be introduced. An LTE-A system is also required to support terminals designed for an LTE system (hereinafter referred to as LTE terminals) in the system in addition to LTE-A terminals.
Additionally, in LTE-A, to achieve an increased coverage, the introduction of radio communication relay apparatus (hereinafter referred to as “relay station” or “Relay Node” (RN)) has been specified (see FIG. 2). Accordingly, the standardization of downlink control channels from base stations to relay stations (hereinafter referred to as “R-PDCCH”) is under way (e.g., see NPL-4, NPL-5, NPL-6, NPL-7, and NPL-8). At present, the following matters are being studied in relation to the R-PDCCH. FIG. 3 illustrates an example of an R-PDCCH region.
(1) A mapping start position in the time-domain of an R-PDCCH is fixed at the fourth OFDM symbol from the beginning of a subframe, and thus does not depend on the rate at which a PDCCH occupies OFDM symbols in the time-domain.
(2) Each R-PDCCH occupies a resource composed of one or more consecutive Relay-Control Channel Elements (R-CCEs). The number of REs forming one R-CCE varies for each slot, or for each reference signal location. Specifically, in slot 0, an R-CCE is defined as a resource region having, in the time direction, a range of from the third OFDM symbol to the end of slot 0, and having, in the frequency direction, a range of 1 RB's width (excluding, however, the region onto which the reference signal is mapped). In addition, in slot 1, an R-CCE is defined as a resource region having, in the time direction, a range of from the beginning of slot 1 to the end of slot 1, and having, in the frequency direction, a range of 1 RB's width (excluding, however, the region onto which the reference signal is mapped). However, proposals have also been made to divide the above-mentioned resource region into two in slot 1, and to have each be one R-CCE.
Further, in LTE, when multiplexing a plurality of terminals for uplink, in order to match timings at which uplink signals transmitted from each of the plurality of terminals arrive at a base station, processing referred to as “Timing Advance” (TA) is performed. The following procedures are executed according to Timing Advance. Namely, a base station measures the timings of uplink signals transmitted from terminals. Based on the measurement results, the base station sends an instruction to specify the transmission timing to each terminal so that the timings at which signals transmitted from the terminals arrive at the base station match between the multiplexed terminals. Subsequently, each terminal transmits an uplink signal at the timing instructed by the base station.